Rubbers for treads of studless tires used for running on ice road surfaces are required to have excellent on-ice and on-snow performances. In order to improve tires in frictional performances on ice road surfaces, for example, the following method has been hitherto adopted: a method of not only using a polybutadiene rubber, which is low in glass transition temperature, or blending a softening agent, such as oil, into a tread rubber to maintain a low hardness of the tread rubber even at low temperatures, thereby improving the rubber in hysteresis frictional performance, but also blending a hard material such as hollow particles, glass fiber or plant granular material thereinto, thereby improving the rubber in scratch frictional performance. In order to improve the tread rubber, particularly, in gripping performance on a wetting road surface (referred to also as a “wet road surface” hereinafter), a method is adopted in which the blend proportion of a filler or a softening agent such as oil is increased in the composition of the rubber. However, a vulcanized rubber obtained therefrom tends to be lowered in low-thermogenic performance and abrasion resistance.
Apart from the above, in the rubber industry, known is the use of a natural rubber wet master batch for improving a rubber composition containing a filler, such as carbon black, in workability when this composition is produced, or in filler dispersibility therein (for example, Patent Document 1 listed below). This is a technique of: mixing, in a liquid phase, a natural rubber latex with a filler-containing slurry solution obtained by mixing a filler and a dispersing solvent with each other at a predetermined ratio beforehand and then dispersing the filler into the dispersing solvent by mechanical force; subsequently adding a solidifier such as an acid to the mixture to solidify the mixture; collecting the solidified product; and then drying the product. The use of the natural rubber wet master batch gives a rubber composition better in filler dispersibility therein and in rubber physical properties such as workability and reinforceability as compared with the use of a rubber dry master batch obtained by dry-mixing a filler with a rubber.
Patent Document 2 listed below describes a rubber composition for ice-and-snow-road tire treads which contains: a natural rubber wet master batch containing a carbon black having a nitrogen adsorption specific surface area of 105 to 155 m2/g, and a CTAB adsorption specific surface area of 100 to 150 m2/g; and a polybutadiene rubber.
Furthermore, Patent Document 3 listed below states that a rubber wet master batch which is one wherein a filler is evenly dispersed and the filler is restrained from being re-aggregated with time, and which is usable as a raw material for a vulcanized rubber excellent in low-thermogenic performance, endurance and rubber strength can be produced by a rubber-wet-master-batch-producing method including step (I) of adding, when the filler is dispersed into a dispersing solvent, at least one portion of a rubber latex solution thereto, thereby producing a slurry solution containing the filler to which rubber latex particles adhere, step (II) of mixing this slurry solution with the rest of the rubber latex solution to produce a rubber latex solution containing the just-above described rubber-latex-particle-adhering filler, and step (III) of solidifying and drying the rubber latex solution containing the rubber-latex-particle-adhering filler.